Field of the Invention
The present invention concerns methods and systems for magnetic resonance (MR) imaging, and in particular concerns such methods and systems for implementing a turbo spin echo (TSE) data acquisition sequence.
Description of the Prior Art
MR imaging is a widely used imaging modality for medical diagnosis as well as for material inspection.
In a magnetic resonance apparatus, the examination object (a patient, in the case of medical magnetic resonance imaging) is exposed to a strong and constant basic magnetic field, by the operation of a basic field magnet of an MR scanner, in which the examination object is situated. The MR scanner also has a gradient coil arrangement that is operated in order to activate gradient fields that spatially encode the magnetic resonance signals. The magnetic resonance signals are produced by the radiation of radio-frequency (RF) pulses from an RF radiator, such as one or more antennas, in the MR scanner. These RF pulses excite nuclear spins in the examination object, and are therefore often called excitation pulses. The excitation of the nuclear spins at an appropriate frequency causes the nuclear spins to deviate, by an amount called the flip angle, from the alignment of the nuclear spins that was produced by the basic magnetic field. As the nuclear spins relax, while returning to alignment in the basic magnetic field, they emit MR signals (which are also RF signals), which are received by suitable RF reception antennas in the MR scanner, which may be the same or different from the RF radiator used to emit the excitation pulse.
The acquired MR signals are digitized and entered into an electronic memory, organized as k-space, as k-space data. The k-space data are also referred to as raw MR data. The k-space memory has a number of individual locations that are available for entering the digitized signal thereat (data entry points), and the path of data entry points along which the digitized data are entered is called the k-space trajectory. The acquired data can be entered into k-space linearly (line-by-line of data entry points) or radially, along a straight or curved path proceeding from the center of k-space toward the periphery of k-space.
Many reconstruction algorithms are known for operating on the k-space data to convert the k-space data into image data representing an image of the volume of the examination object from which the raw MR data were acquired. This reconstruction algorithm is executed in an image reconstruction computer, resulting in an image data file from the computer that can be shown at the display screen of a monitor, or archived for storage.
After the nuclear spins have been flipped by the RF excitation pulse, the resulting MR signal exhibits an exponential decay in strength as the excited nuclear spins relax. This decaying signal is referred to as an echo signal, or simply as an echo. A commonly used data acquisition sequence of appropriately timed RF excitations and gradient pulse activations (switchings) is the echo planar imaging (EPI) sequence. In an EPI sequence, instead of measuring only one echo after each excitation pulse, multiple echoes are detected by multiple activations of the readout gradient after a single excitation pulse. In an EPI sequence, therefore, the total echo time of the decaying MR signal that follows the excitation of the nuclear spins is divided into a number of individual echo times, corresponding to the number of activated readout gradients.
Another sequence which acquires multiple echoes after each excitation pulse is known as the turbo spin echo (TSE) sequence. Here, multiple refocusing RF pulses are radiated that continually refocus the decaying magnetization, and an individual echo signal is respectively acquired after each refocusing pulse. The repeatedly activated refocusing pulses slow the decay of the magnetization, so that the individually detected echoes have a higher signal strength than the echoes acquired in an EPI sequence.
In a sequence known as TurboGSE (TGSE), additional gradient echoes are generated before and after each spin echoes. The spin echoes are allocated to the center of k-space, in order to produce pure T2 contrast. If multiple image lines are obtained during a single echo, the imaging pulse sequence type is a TGSE pulse sequence.
The (effective) echo time (TE) is the time between the RF excitation pulse and the acquisition (sampling) of the MR signal of the k-space center. The repetition time (TR) is the time between two successive RF excitation pulses. By appropriately selecting TR and TE, the acquired MR signal can be differently weighted. In general terms, a sequence with a long TR and a short TE is usually called proton density (PD)-weighted, a sequence with a short TR and a short TE is usually called T1-weighted, and a sequence with a long TR and long TE sequence is usually called T2-weighted. (The physical reasons that result in the naming of these different types of weighting are not relevant to the discussion herein.)
The type of weighting that is selected, in general terms, determines which MR signal source (i.e., a source from which detected MR signals originate) will appear brighter in the reconstructed image, and which MR signal source will appear darker. Thus the selected weighting determines the contrast with which a particular MR signal source will be visually represented in the reconstructed image. In the case of medical MR imaging, the respective signal sources are different types of tissue, and therefore the type of weighting is selected dependent on the tissue that is desired to be shown with the best contrast in the reconstructed image, which is in turn dependent on the medical diagnosis that is desired to be made by evaluation of the reconstructed image.
In clinical practice, there is often the need to acquire the same image with the identical sequence type, but with different echo times. One application is the PD/T2 TSE sequence, wherein a PD-weighted image dataset and one or more T2-weighted image datasets are obtained, such as for head, abdomen or joint imaging. Another application is T2 mapping, making use of a TSE sequence.
A conventional TSE sequence with different echo times combines two or more separate readout blocks back-to-back to each other, after a single excitation pulse, which produces one echo train. The sequence diagram for such a conventional TSE acquisition with two contrasts (TE1=89 ms and TE2=169 ms) with an echo train length of 16 for each contrast is shown in FIG. 1. After the MR data in the first block have been acquired (in this case with a linear k-space reordering scheme), the MR data for the second block are acquired, independently of the first block (in this case with a centric out k-space reordering scheme). This results in a relatively long total echo train length of 32.
As a consequence of these separated blocks, there is only limited freedom in selecting the echo times, because each echo time must fit into its respective block. In the example shown in FIG. 1, this means that TE1 can be selected in a range between 9 to 159 ms, while TE2 can be selected between 169 and 329 ms. If a shorter time between the two echoes is desired (for example, TE1=59 ms, TE2=99 ms), a shorter echo train must be selected, thereby further prolonging the total acquisition time.
Another disadvantage that results from this use of a conventional TSE sequence results from different modulation transfer functions (MTF) and point spread functions (PSF) that describe the filtering that acts on the image data, depending on the aforementioned reordering of the k-space lines. The PSF is the equivalent of the MTF in the image domain. If the TEs for the different contrasts are not placed exactly at the same positions within the blocks, the MFT, and thus the PSF, will differ for the different contrasts. In the example of FIG. 1, if TE1 is at the beginning of the first acquisition block, the reordering must be centric out, which will lead to stronger filtering and thus a reduced spatial resolution. If TE2 is at the end of the second acquisition block, the reordering must be reversed centric, which will lead to stronger side lobes in the PSF, and thus a more pronounced phenomenon known as Gibbs ringing. Apart from different image impressions (visualizations) this leads to inaccurate results in T2 fitting, because the respective intensities of the individual pixels of the respective contrast images are altered differently by the different MTFs. An example of MTFs for centric reordering and reversed centric reordering is shown in FIG. 2A, and the corresponding PSFs for centric and reversed centric reordering are shown in FIG. 2B.
A technique for accelerating MR data acquisition, and thereby reducing the time that the patient must spend within the MR scanner, is the simultaneous multi-slice (SMS) technique. Details of techniques for SMS data acquisition can be found in the article by Setsompop et al., “Blipped-Controlled Aliasing in Parallel Imaging for Simultaneous Multi-Slice Echo Planar Imaging with Reduced g-factor Penalty,” Magnetic Resonance in Medicine, Vol. 67(5), pp. 1210-1224 (2012). In general, the SMS technique involves the simultaneous excitation of a number of slices of a volume that is to be imaged, and the simultaneous acquisition of the resulting MR signals from the multiple slices. The number of slices that are simultaneously excited are typically grouped in a slice group. The SMS technique is typically characterized by an acceleration factor that indicates how many slices are simultaneously excited in a slice group by one individual radio-frequency pulse. The acceleration factor is at least two.
Since the k-space data for all of the simultaneously acquired slices are entered simultaneously into k-space, the subsequent reconstruction of the respective images of the individual slices involves a separation of the k-space data for the multiple slices. The data for the respective slices can be separated using a known slice separation method, such as the slice GRAPPA (Generalized Autocalibration Partially Parallel Acquisitions) technique, noted in the aforementioned article by Setsompop et al. This enables the SMS technique to be used to acquire a number of slices at the same time. A schematic illustration of this known technique is shown in FIG. 3, for the example of three slices S1, S2 and S3.
As noted above, in SMS imaging, multiple slices are excited at the same time. In the example of FIG. 3, this takes place using blipped CAIPIRINHA (Controlled Aliasing in Parallel Imaging Results in Higher Acceleration), and is described in the aforementioned article by Setsompop et al. (shortened to blipped CAIPI therein)
In SMS imaging, in order to excite multiple slices simultaneously, a multi-band (MB) excitation pulse is used. For each slice in which the nuclear spins are to be excited, a linear phase ramp is added to a standard excitation pulse. The linear phase results in a slice shift in the spatial domain. The pulse shapes for all bands are added (superimposed), resulting in a baseband-modulated MB pulse.
The individual signals that are superimposed as a result of the MB pulse are received from respective individual coils of the MR data acquisition scanner. These coils necessarily respectively occupy different spatial positions, but are in close enough proximity to each other so that the same nuclear spins will be detected by more than one of the multiple coils. Because each coil is situated at a different position in space, however, the effect of each individual coil on the reception of a given individual nuclear spin will be slightly different, and must be taken into account. This is done by calculating a so-called g-factor (geometry factor) for the coil array that is used. The extent to which the g-factor degrades the resulting reconstructed image is called the g-factor penalty. In order to reduce the g-factor penalty in SMS imaging, interslice image shifts are deliberately induced during the readout in blipped CAIPI, either by gradient blips on the slice axis or by modulating the phase of the RF pulses. After the data have been acquired, the simultaneously excited slices are collapsed into a single slice for entry of the data into k-space. The individual slices can be separated in the post-processing, utilizing the aforementioned slice GRAPPA technique, as schematically illustrated in FIG. 3.